1. Field of the Invention
This invention relates to a fixing device for use in an image formation apparatus such as an electrophotographic copying apparatus or recording apparatus, and more particularly to a fixing device in which a sheet such as paper having toner image on the surface thereof is passed between a pair of rollers pressed against each other to thereby fix the toner image on the sheet.
2. Description of the Prior Art
Description will hereinafter be made with a pressure fixing device taken as an example of the fixing device.
In the conventional pressure fixation, there have been made various propositions such as increasing the accuracy of the surfaces of rollers and applying a pressure thereto to improve the fixativeness, or making the surfaces of rollers smooth or coating the rollers with polyamide resin or providing a coating on paper to smooth the paper on which an image is to be fixed. However, in such pressure fixing devices, it has been necessary to apply a linear pressure of 20 Kg/cm or higher, or often 30 Kg/cm or higher, and effect fixation while varying the shape or thickness of paper, for example, by about -10%, under such pressure.
Generally, the surface of paper has a number of concavities. These are the interstices between the fibers of the paper. The depth of such concavities of the paper is of the order of 10.mu. and the distance between adjacent concavities is of the order of 20 to 50 .mu.m.
When a toner image is transferred onto such paper, as schematically shown in FIG. 1 of the accompanying drawings, part of the toner comes into the concavities on the surface of the paper 1 and part of the toner rides onto the fibers of the paper. In FIG. 1, reference numeral 2 designates the toner which has come into the concavities of the paper 1, and reference numeral 3 denotes the toner which has ridden onto the fibers of the paper. Reference character h indicates the depth of the concavities which is about 10 .mu.m. Also, l indicates the distance between adjacent concavities which is of the order of 20 to 50 .mu.m.
When this paper is passed between the rollers to fix the toner image on the paper, as shown in FIG. 2 of the accompanying drawings, the surface of the paper which has originally been in a position indicated by broken line is deformed by a pressure into a shape as indicated by solid line. As seen in FIG. 2, the toner 2 which has come into the concavities of the paper has no pressure applied thereto and is not fixed.
On the other hand, the toner 3 which has been on the fibers of the paper is subjected to a pressure by the roller 5 and is thereby crushed and fixed as shown.
In order that pressure may be applied even to the toner 2 which has come into the concavities of the paper and such toner 2 may be fixed, a higher pressure becomes necessary. Generally, it is 20 Kg/cm or higher and, in order to obtain sufficient fixativeness, it must be 30 Kg/cm or higher. Also, where there is present toner 7 between toner particles 6 of a larger particle size as shown in FIG. 3, the toner 7 will have no pressure applied thereto and will remain unfixed if a sufficient pressure to dilate only the toner 6 is applied by the roller 5 as shown in FIG. 4.
To eliminate the above-described condition, the concavo-convexity of the paper must be deformed by a pressure so that such concavo-convexity is eliminated. For this purpose, it would be necessary to apply a linear pressure of at least 20 Kg/cm or higher and, in most cases, a linear pressure of 30 Kg/cm or higher. However, if such a high pressure is applied, the surface of the paper may undesirably be lustered to reduce the quality of image and the paper may be curled.
Also, as mentioned previously, there is a method of providing a coating on the paper and thereby smoothing the surface of the paper to enhance the fixativeness, whereas this is not enough to obtain sufficient fixativeness and the quality of the paper is varied by the coating and cost is increased. Further, if the paper is coated with polyamide resin, such resin which is hard will not sufficiently follow the concavities of the paper and fixation will become insufficient.